1. Field
The present disclosure relates to a hot-pressing device and a method of manufacturing a hot-pressed product, and in particular, to a hot-pressing device and a method of manufacturing a hot-pressed product with which a workpiece is press-formed while simultaneously quench-hardened by using a coolant.
2. Description of the Related Art
There is a hot-press-forming method for obtaining a high-strength and high-precision press-formed product by press-forming a workpiece, such as a steel plate, by using a die while simultaneously quench-hardening the workpiece with the die. Japanese Unexamined Patent Application Publication No. 2002-282951 describes a technology for quench-hardening a workpiece by removing heat not only through contact between the workpiece and a die but also through contact between the workpiece and a coolant.
Japanese Unexamined Patent Application Publication No. 2002-282951 discloses a press-forming device that uses water, an aqueous solution dispersed with synthetic particles, or the like as a coolant. The press-forming device includes a plurality of coolant grooves that are formed in a forming surface of a die (or a punch) so as to be arranged side by side with predetermined distances therebetween and a coolant circulation device that supplies the coolant to and recovers the coolant from the coolant grooves (see paragraphs [0024] and [0027], and FIGS. 2 and 3). In a hot-press-forming process performed by using the press-forming device, the punch is maintained in a state in which the punch has reached a bottom dead center position. In this state, the coolant is circulated into spaces formed between the coolant grooves and the workpiece, thereby cooling the workpiece so as to quench-harden the workpiece. The punch is maintained at the bottom dead center at least until the temperature of the workpiece has decreased to a predetermined value.
In the cooling process, both the die and the coolant remove heat from the workpiece. To be specific, in areas of the forming surface of the die in which the grooves are not formed, the die directly contacts the workpiece and removes heat from the workpiece. In areas of the forming surface in which the grooves are formed, the coolant contacts the workpiece and removes heat from the workpiece.
In the die of the hot-pressing device described in Japanese Unexamined Patent Application Publication No. 2002-282951, an inlet for introducing the coolant into the coolant grooves is located at a middle portion of the forming surface of the die. Outlets for discharging the coolant from the coolant grooves are located on both sides of matching surfaces of the die and the punch (see FIG. 2 of Japanese Unexamined Patent Application Publication No. 2002-282951). That is, the coolant, which is introduced into the coolant grooves through the inlet, flows along a bottom wall, side walls, and flange portions of the workpiece; and the coolant is discharged from the outlet (see paragraph [0037]).
Therefore, for the die of the hot-pressing device described in Japanese Unexamined Patent Application Publication No. 2002-282951, the path length along which the coolant flows in contact with the workpiece is comparatively large. Accordingly, there is a large difference between the temperature of the coolant that has just been introduced into the coolant grooves and the temperature of the coolant immediately before being discharged. Therefore, the farther a portion of the workpiece is from the inlet, the lower the cooling efficiency of the coolant at the portion. As a result, a defect due to nonuniform cooling may occur, because different portions of the workpiece are cooled to different degrees.
As the cooling efficiency decreases, the time required to quench-harden a workpiece increases. Therefore, it becomes difficult to increase the productivity by decreasing the time for which the workpiece is held in the die. When the workpiece is nonuniformly cooled, different portions of the workpiece may be quench-hardened to different degrees, and the precision of the dimensions of a hot-pressed product obtained through the hot-press-forming process may decrease.